Hi Linus,
On Tue, Jun 7, 2022 at 8:15 PM Linus Torvalds
<torva...@linux-foundation.org> wrote:
> On Tue, Jun 7, 2022 at 3:23 AM Geert Uytterhoeven <ge...@linux-m68k.org>
> wrote:
> > These header files are heavy users of large constants lacking the "U"
> > suffix e.g.:
> >
> > #define NB_ADAPTER_ID__SUBSYSTEM_ID_MASK 0xFFFF0000L
>
> As Andreas says, this is not undefined behavior.
>
> A hexadecimal integer constant will always get a type that fits the
> actual value. So on a 32-bit architecture, because 0xFFFF0000 doesn't
> fit in 'long', it will automatically become 'unsigned long'.
>
> Now, a C compiler might still warn about such implicit type
> conversions, but I'd be a bit surprised if any version of gcc actually
> would do that, because this behavior for hex constants is *very*
> traditional, and very common.
>
> It's also true that the type of the constant - but not the value -
> will be different on 32-bit and 64-bit architectures (ie on 64-bit, it
> will be plain "long" and never extended to "unsigned long", because
> the hex value obviously fits just fine).
>
> I don't see any normal situation where that really matters, since any
> normal use will have the same result.
>
> The case you point to at
>
>
> https://lore.kernel.org/r/cak8p3a0qrihbr_2fq7uz5w2jmljv7czfrrarcmmjohvndj3...@mail.gmail.com
>
> is very different, because the constant "1" is always just a plain
> signed "int". So when you do "(1 << 31)", that is now a signed integer
> with the top bit set, and so it will have an actual negative value,
> and that can cause various problems (when right-shifted, or when
> compared to other values).
>
> But hexadecimal constants can be signed types, but they never have
> negative values.
Thank you, I stand corrected.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
